At What Point In The Story Do You Find Montresor Disturbing

Montresor is the most disturbing when he leaves Fortunato behind while he is screaming. 

Montresor is disturbing during most of the story.  At the beginning, he explains to us that he needs to kill a guy for a reason that is important to him but apparently not important enough to tell us. 

I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong. 

Although this is really disturbing, because he is plotting a cold-blooded murder and his only concern is that he gets away with it, it gets worse as he slowly reels poor Fortunato in with reverse psychology.  First he tells him that he has a really cool cask of wine that he needs an opinion on, but he is going to ask someone else instead.  Then he gets him underground, and keeps inquiring about his health and saying they should go back. 

Come,” I said, with decision, “we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there isLuchesi—” 

This is definitely disturbing.  It shows that Montresor is conniving and heartless.  However, the most disturbing action occurs when he actually gets his prey holed up in the wall.  At this point, Fortunato is drunk and perhaps congested to the point of being lightheaded.  Either way, he is not aware of what is going on until it is too late.  Montresor gets him bricked up before he realizes he is being buried alive. 

Fortunato thinks it is a joke at first, or hopes it is. 

“He! he! he!—he! he! he!—yes, the Amontillado. But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest? Let us be gone.”

“Yes,” I said, “let us be gone.”

To me, this is the most disturbing part.  Montresor has just gotten creepier and creepier, but his attitude at this part of the story is bone-chilling.  He bricks a man up in a wall, and then says "let us be gone" and leaves him there?  How messed up is that?

Poe builds this story up very carefully, so that we can see how demented Montresor is from the very beginning.  At the end he tells us fifty years have gone by, but you have to wonder who he's talking to.